"Wayne" wrote in
:

Many years ago, I might have been able to somehow deduce cable losses
from a Smith chart, but nowadays I just use the published curves.

An email correspondent suggested the radially scaled "SW LOSS COEFF"
gives a direct reading loss under mismatch from the chart.

There are assumptions that underly that scale, pre-requisites if you
like for it to be accurate. They are explained in Smith's book: "If a
waveguide is one or more wavelengths long, the average increase in
dissipative loss due to standing waves in a region extending plus or
minus one-half wavelength from the point of observation may be expressed
as a coefficient or factor of the one-way transmission loss per unit
length".

So, it does not apply to a feed line of less than a full wave, or any
residual of less than a full a wave after working out the additional
loss in each of the full wave segments of a line. It is tedious and
accurate only for lines of integral full waves length.

The potential error applying it to arbitrary line length ranges from
insignificant to huge.

The graphs in the ARRL suffer the same problem, and IIRC they do not
state the assumptions.

The Smith chart solution I described in my earlier post works for
arbitrary line lengths, and properly reveals the cases where loss under
standing waves is less than matched line loss. TLLC
(http://www.vk1od.net/calc/tl/tllc.php) also calculates the loss
correctly for arbitrary line lengths.

Owen

On Mon, 02 Jan 2012 01:39:48 -0800, Wayne wrote:

Still having fun playing with my 8.5 foot whip on the metal patio roof,
with coax feedline to an antenna tuner in the shack. Operating on
15,12, and 10.

The antenna tuner is a typical t network with series capacitors and an
inductor to ground. I was unable to get a match on 15 meters with the
antenna tuner. So, I thought I would add more coax to the feedline and
move around the SWR circle on the Smith chart. Apparently this provides
a set of matching parameters that the antenna tuner can now cope with.

I don't have instruments to measure many parameters, but a rough guess
is that the 15 meter SWR is at least 20:1.

So the question is: are there parts of the Smith chart that are going to
be more difficult to match, and should be avoided?

Have you ever compared the field strengths above and below the steel
cover?

"Wond" wrote in message ...

On Mon, 02 Jan 2012 01:39:48 -0800, Wayne wrote:

Still having fun playing with my 8.5 foot whip on the metal patio roof,
with coax feedline to an antenna tuner in the shack. Operating on
15,12, and 10.

The antenna tuner is a typical t network with series capacitors and an
inductor to ground. I was unable to get a match on 15 meters with the
antenna tuner. So, I thought I would add more coax to the feedline and
move around the SWR circle on the Smith chart. Apparently this provides
a set of matching parameters that the antenna tuner can now cope with.

I don't have instruments to measure many parameters, but a rough guess
is that the 15 meter SWR is at least 20:1.

So the question is: are there parts of the Smith chart that are going to
be more difficult to match, and should be avoided?

Have you ever compared the field strengths above and below the steel
cover?
-
No I haven't. What would be the practical tests? Example, measuring FS at
x feet from the antenna at y height above the cover, vs below the cover at
what point?

On Mon, 02 Jan 2012 08:47:29 -0800, Wayne wrote:

"Wond" wrote in message ...

On Mon, 02 Jan 2012 01:39:48 -0800, Wayne wrote:


Have you ever compared the field strengths above and below the steel
cover?
-
No I haven't. What would be the practical tests? Example, measuring FS
at x feet from the antenna at y height above the cover, vs below the
cover at what point?

I was thinking, it would be a rough indication what proportion of
power is radiated from the feedline.

"Wond" wrote in message ...

On Mon, 02 Jan 2012 08:47:29 -0800, Wayne wrote:

"Wond" wrote in message ...

On Mon, 02 Jan 2012 01:39:48 -0800, Wayne wrote:


Have you ever compared the field strengths above and below the steel
cover?
-
No I haven't. What would be the practical tests? Example, measuring FS
at x feet from the antenna at y height above the cover, vs below the
cover at what point?

I was thinking, it would be a rough indication what proportion of
power is radiated from the feedline.
-
Good point, and I'm not sure that the feedline is currently in optimal
configuration.

The feedline lies on the metal roof to the edge, where there is a small 8
turn loop. Then it goes another 6-8 feet to the tuner.

My thoughts are that your antenna theory has holes in it.
If you want it to tune on 15 meters, it needs to be a fraction of a wavelength long on 15 meters - which a antenna tuned for 10 meters has a real hard time sometimes tuning up on 12 or 15 meters.

Instead of trying to redesign the mouse trap, why not invest your money towards a used 11 meters chicken band antenna.
A antenna - regardless of who makes it or what they market it for - is a antenna.

By shortening a 11 meter antenna a couple of inches, it will work wonderfully for 10 meters.

The suggested length of coax with solid dielectric .66 velocity factor usually comes out somewhere around 18 feet long for a mobile, so working with the Smith Chart will get your SWR - line length to a better SWR, but to make it resonant - it needs to be a certain size.

You would be better off designing the antenna for 15 meters and then using the transmatch to tune it down to 10 meters then to try to use a 10 meters antenna and try to tune it up for 15. Then again, your antenna really isn't any good for either band in my opinion, due to the fact that a 5/8ths wave antenna probably works best in most situations for a base station radio for 10 meters and your antenna is way too short.

Even a 1/4 wave antenna at 15 meters would probably need to be at least 11.5 feet long, with a nominal length of about 31 feet using a vertical antenna.